A number of advances in internal combustion engine technology include the application of solenoid-driven hydraulic valve actuators to provide more precise control over a number of engine functions such as control of the exhaust valves and fuel injectors. Hydraulic actuators used in applications such as these require the actuator to accelerate rapidly (have high axial output force), to be extremely reliable (i.e., operate for a great number of cycles during the life of the actuator assembly), and to have very low hysteresis.
Thus, there is a need for solenoid-driven actuators which are fast-acting and precisely controllable and which have long operational lives. Also, there is a more general need for improved solenoids for a variety of solenoid applications. Various solenoid structures have been created with certain of these objectives in mind, and examples will be discussed briefly at the end of this background section.
In considering what may be necessary to achieve the desired performance, it is important to minimize the gap in the magnetic circuit between the armature of the solenoid and the surrounding structure of the solenoid which shapes the magnetic field acting on the armature. Such a configuration increases the axial output force of the solenoid, which in turn causes the actuator driven by the solenoid to accelerate more quickly.
At the same time, however, in order to prevent failure of the actuator driven by the solenoid, it is extremely important to minimize the lateral forces (side forces) on the moving components of the hydraulic valve actuator. This minimizes wear of the actuator components. Achieving long life can be accomplished by the combination of precise bearing support for the armature of the solenoid and minimizing the transmission of any residual lateral forces from the armature to the driven component of the actuator.
In addition, the static friction of the armature within the solenoid should be kept to a minimum in order to reduce or eliminate hysteresis. (In this context, hysteresis refers to the unwanted lag in the commanded displacement of the armature due to static friction between the armature and the core of the solenoid.) Achieving low static friction can be accomplished by reducing the bearing contact surface and, of course, by constructing the bearing contact surfaces out of low static friction materials.
The engineering reasoning discussed above undergirds the present invention. The present invention addresses all of the needs described above and thus provides an improved solenoid and solenoid-driven hydraulic valve actuator with high axial driving forces, precise performance, high reliability, and long life.
Examples of prior solenoids and hydraulic valve actuators with characteristics of interest include U.S. Pat. Nos. 5,435,519, 5,467,962 and 5,494,255, which disclose various solenoid-driven valve actuators. The solenoids of these devices each have a fairly small gap between a pole piece and the armature. In each case, however, there is a long bearing surface which, as discussed above, can tend to add unwanted static friction, leading to possible hysteresis. In certain of these prior devices, because of the nature of the bearing materials, the fairly small gaps (between the pole pieces and the armatures) are not as small as may be desired to maximize solenoid performance.
Despite various developments in the past, given the extreme performance specifications required of components for high-performance engines there remains a need for improved solenoids and solenoid-driven actuators which are fast-acting and precisely controllable and which have long operational lives.